The present invention relates to a centrifuge control system and method and, more particularly, to a control system and method for controlling the operation of a decanting centrifugal separator in response to variations in several operating parameters.
Decanting centrifuges are well known in the art and are designed to process a mixture of two constituents, usually a liquid and a solid, and to separate one from the other. These types of centrifuges feature a rotating bowl and a spiral screw conveyer disposed inside the bowl which rotates in the same direction as the bowl and at a different speed. The mixture, which for the purpose of example, will be assumed to be a liquid having relative fine solid particles entrained therein, enters the bowl and the centrifugal forces direct and hold it against the inner wall of the bowl in a xe2x80x9cpoolxe2x80x9d while the fluid is displaced to one end portion of the bowl for discharge. The solid particles settle against the wall and are transported, or displaced, by the screw conveyor to discharge ports extending through the opposite end portion of the bowl for discharge. Typical applications of this type of centrifuge is in pulp, paper, and waste water treatments and for the removal of dirt, sand, shale, abrasive cuttings, and/or silt particles (hereinafter referred to as xe2x80x9csolid particlesxe2x80x9d) from drilling fluid after the fluid has been circulated through a drilling bit to lift the cuttings to the surface in an oil field drilling operation.
However, there are several parameters involved in the operation of a centrifuge, such as bowl speed and torque, conveyor speed and torque, fluid pump rate, fluid viscosity or dilution, and fluid solids content and properties. Since the operational goals of the centrifuge itself are fairly precise, it is important that the centrifuge be controlled so that its operation is optimized in response to variations in the above parameters. Also, the centrifuge itself can be operated in different modes in accordance with different design goals, such as maximum solids separation, maximum solids discard volume, etc., which requires further precise control. Therefore, the centrifuge should be controlled in a manner so that precise predetermined operational modes can be maintained despite variations in the various operational parameters and design goals. Such a control system and related method are disclosed in U.S. Pat. No. 5,857,955, assigned to the assignee of the present application. Although this system is eminently successful in maintaining precise predetermined operational modes despite variations in the various operational parameters and design goals it is difficult to insure that the density of the mixture is within a predetermined acceptable range which is important to avoid excessive loading of the conveyor and/or the bowl. Also, it would be advantageous if the system disclosed in the above patent would react much more quickly and efficiently to changes in the properties of the fluid stream entering the centrifuge, and if the mass rate and density of the separated fluid discharging from the bowl could be estimated.
The present invention, accordingly, provides a system and a method for controlling a centrifuge system including a rotatable bowl and a rotatable screw conveyor extending in the bowl. A variable speed drive pump pumps a mixture of liquid and solid particles to the bowl and two drive assemblies respectively rotate the bowl and the conveyor to separate the solids from the liquids. A computer is connected to the drive assemblies and to the pump for receiving signals from the drive assemblies based on the rotation of the bowl and or the conveyor and for sending signals to the pump to control the volume of mixture pumped from the pump to the bowl.
As a result, major advantages are achieved with the system and method of the present invention. For example, the computer will process the above signals and control the drive units for the pump, to insure that the density of the mixture is within a predetermined acceptable range. Also, this automatic control of the bowl and the conveyor in conjunction with automatic control of the pump will enable the system to react much more quickly and efficiently to changes in the properties of the fluid stream entering the centrifuge. Also, the computer can be provided with software to enable it to estimate the mass rate and density of the separated fluid discharging from the bowl.